TRIOSMIUM CLUSTERS WITH 2-PYRIDYLPHOSPHINES AS LIGANDS

Room-temperature reactions of [Os3(CO)10(MeCN)2] with the tertiary phosphines (L), diphenyl(2-pyridyl)phosphine, phenyldi(2-pyridyl)phosphine, and tri(2-pyridyl)phosphine, led to clusters of the type [Os3(CO)11L] 1. [Os3(CO)10L2] 2, and [Os3(CO)10(mu-L)] 3 which is the major product in each case. Clusters 1 and 2 have ligands L co-ordinated through the phosphorus atom alone, whereas 3 contain diaxially co-ordinated bridging ligands through one P and one pyridyl substituent. Two isomers of 3 [L = PPh(2-C5H4N)2] are in rapid exchange in solution above room temperature and the clusters 3 [L = PPh2(2-C5H4N) or P(2-C5H4N)3] gain a time-averaged plane of symmetry by a process which is the counterpart of this isomerisation; dynamic transfer of pyridine co-ordination between two metal atoms is proposed on H-1, C-13-{H-1} and P-31-{H-1} NMR evidence. Protonation of 3 [L = PPh2(2-C5H4N)] arrests this process. Thermal treatment of 3 [L = PPh2(2-C5H4N)] led to a major product [Os3(sigma-Ph){mu3-PPh(2-C5H4N)}(CO)9] (60%) (crystal structure reported) which contains the five-electron donating phenyl(2-pyridyl)phosphido bridge and a terminal phenyl ligand. Minor products are the cluster [Os3(mu-C5H4N)-(mu-PPh2)(CO)10] (6%) (crystal structure reported), [Os2(mu-C5H4N)(mu-PPh2)(CO)9] (trace), which is derived from the former, and [Os3(mu-PhCO){mu3-PPh(2-C5H4N)}(CO)9] (trace). These clusters are generated by cleavage of either phosphorus-phenyl or phosphorus-2-pyridyl bonds; cleavage of a bond within the five-membered ring forming the bridge is less favoured than that of an exocyclic substituent.